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State of our air

About the indicators

In 2014 Air domain report we presented three national indicators of air quality. This page provides the background, supporting information, methodology, and limitations for the data used in the report. See Criteria for selecting our environmental indicators on how indicators are selected.

Estimated total emissions (tail pipe and brake and tyre wear, in kilotonnes) from on-road vehicles for these pollutants:

carbon monoxide

volatile organic compounds

nitrogen oxides

PM10

PM2.5.

Methodology

This indicator uses a model developed for the New Zealand Transport Agency and Auckland Council to estimate emissions from New Zealand’s vehicle fleet over time. The model considers the types of vehicles on the road, their fuel efficiency, and the distances and speed they travel. See NIWA’s Indicators for Environmental Domain Reporting report for more information. Information on accessing the model is available from the New Zealand Transport Agency website.

The model is based on internationally recommended approaches, uses internationally recognised emission factors, and is validated with road testing results. However, limited validation of the model outputs for heavy diesel vehicles exist. See section 2.6 of the following report for further information. See NIWA’s Indicators for Environmental Domain Reporting report for more information.

Modelling assumptions and parameters are reviewed periodically to ensure the model best reflects the emissions from on-road transport at a given point in time. Any changes to the model are peer-reviewed internationally.

The modelling assumes that the vehicle fleet composition is the same throughout New Zealand as the regional information is of variable quality.

Estimated pollutant emissions do not always equal the measured concentrations. Other factors also contribute to concentrations, such as weather conditions.

Additional information to understand the link between the topic and the indicator

Only national on-road transport is included. Off-road transport, including sea, air, and rail transport, is excluded.

Changes to time series

None

Indicator

National annual average PM10 concentrations

Related topic

Concentration of particulate matter (state)

Indicator definition

A population weighted annual average PM10 concentration indicator from both natural and anthropogenic (human-made) sources.

The long-term (annual) average concentration takes into account both peak and low pollution periods and gives an indication of long-term health risks. Annual averages also represent a larger area and population than short-term (daily) measurements.

Background

Exposure to high PM10 concentrations are linked to adverse health effects such as lung and heart conditions. PM10 is the measured air pollutant that most frequently exceeds national and international thresholds in New Zealand.

Presentation of indicator components

Population weighted annual average concentration of PM10 (micrograms per cubic metre).

Methodology

The national annual average PM10 concentration is estimated using regional council and unitary authority monitoring stations data (54 sites in 2012) and Statistics NZ’s population estimates.

The national average is calculated by using the average concentration for each monitoring site, weighted by the population each site represents.

The population represented by each monitoring site is considered to be the population of the urban area the monitoring site is located in. Where multiple monitoring sites are located within an urban area, the population of the urban area is divided across each of the monitoring sites.

This approach accounts for the difference in the distribution of monitoring sites within New Zealand relative to the represented population. For example, in 2012, 40 percent of monitoring sites were in small towns but populations in small towns represent much less than 40 percent of the total population. An average of all monitoring results would be influenced more by the concentrations in small towns than the proportion of the population it represents.

Monitoring information is only included if the site achieves greater than 75 percent valid data collection and follows good practice approaches (Ministry for the Environment, 2009). This helps ensure the data is representative of the location.

Rural areas and some urban areas are not included as no air quality monitoring occurs at these locations or the population at the monitoring location is unknown. These unmonitored areas are approximately 25 percent of New Zealand’s population. Currently there is no robust method to estimate air quality concentrations in these areas. This is an improvement that will be considered for future reporting.

Some of the monitoring occurs at sites expected to have the highest concentrations (eg close to road traffic or peak urban areas), and so may not be representative of the whole population it is assumed to represent.

Additional information to understand the link between the topic and the indicator

Annual average concentration of PM10 is an indicator of long-term concentration. Short-term indicators are measured by daily concentrations. Long-term concentrations give a better indication of the impact on public health and provide greater representation of area and population than short-term (daily) measurements.

Changes to time series

The sample includes monitoring sites achieving 75 percent valid data. The sample of monitoring sites varies year to year, with additional sites being included if best practice approaches are followed, or omitted if not of adequate standard.

Indicator

National-level health impacts due to exposure to PM10

Related topic

Public health effects (impact)

Indicator definition

The estimated number of premature deaths, hospitalisations, and restricted activity days for the New Zealand population from exposure to sources of PM10 from human activities.

Background

Exposure to PM10 is linked to adverse health effects such as lung and heart conditions. Estimates can be made on the impact exposure to PM10 is having on the health of New Zealanders. Tracking these estimates over time shows whether population health impacts due to PM10 concentrations from human activities are improving or worsening.

Presentation of indicator components

Estimated number of premature deaths due to PM10 exposure.

Estimated number of hospitalisations due to PM10 exposure.

Estimated number of restricted activity days due to PM10 exposure.

Methodology

Heath impacts due to air pollution are estimated by determining the concentrations of the pollutant the population is exposed to and the probability of health risks after exposure to concentrations of that pollutant.

Information on the PM10 concentrations the population is exposed to comes from regional council and unitary authority monitoring stations data and the census.

Dose response functions are calculated from international and national epidemiological studies.

The focus is on anthropogenic (human-made) sources of PM10 as these sources can be managed and abated. Information on the anthropogenic component of PM10 comes from GNS Science.

Health effects from PM2.5 exposure are not measured as there is insufficient monitoring information to do so.

Data for 2006 are based on monitoring data from 2006 to 2008, while 2012 data are based on monitoring data from 2010 to 2012. The time period reflects the year of the census or the closest to it for which monitoring data is included. Results are based on the average concentration over a three-year period. This provides an indicative estimate of the long-term health effects by smoothing out short-term extremes caused, for example, by a particularly warm or cold winter.

Health and air pollution in New Zealand model, 2012 (model framework).

Regional councils and unitary authorities (PM10 monitoring data).

Census (population estimates).

GNS Science (natural PM10 concentrations).

Limitations to data and analysis

Concentrations of PM10 are imputed in census area units where monitoring has not occurred.

Location-specific anthropogenic PM10 component information was applied where available. For other locations, a national average anthropogenic component estimate (based on the location-specific information) was used.

The most relevant dose response functions based on national and international studies are used.

The base incidence rates for health outcomes were not updated as new data was not available. This is expected to have a minor impact on the results – an increase or decrease in premature mortality and hospitalisations by approximately 1.5 percent. Restricted activity days are not influenced by base incidence rates.

Additional information to understand the link between the topic and the indicator

The estimates reflect only the public health effects of PM10 from human activities and do not show the effects of other pollutants that have adverse health effects. Therefore, this indicator understates the total effect of air quality on public health.

Changes to time series

The sample includes monitoring sites achieving 75 percent valid data. The sample of monitoring sites varies year to year, with additional sites included if best practice approaches are followed, or omitted if not of adequate standard.